
#include <stdio.h>
#include <math.h>

int main(int argc, char** argv) 
{
	char line[80];
	int i, value;
	float total = 0;

	// read P3, size, max value
	//scanf("%s\n", line);
	//scanf("%s\n", line);
	//scanf("%s\n", line);

	// Start reading the values
	for (i=0; i<640*480*3; i++) {
		scanf("%d", &value);
		total += value*value;
	}
	total = total / (680*480*3);

	printf("rms : %f\n", sqrt((double) total)/255.0);
	return 0;
}

